1. Field of the Invention
This invention relates to a method of controlling an aluminum surface cleaning composition. More specifically it is concerned with a method whereby it is possible, both easily and effectively, to monitor, control and thus maintain the effectiveness of an acidic cleaning solution used to remove lubricant oil and so-called "smut" (aluminum powder abraded from the surface) which adheres to the surface of aluminum articles after their manufacture by metal-forming operations.
2. Description of the Related Art
Aluminum articles such as beverage containers made of aluminum or aluminum alloys are customarily manufactured by a metal-forming operation called "drawing and ironing", often and conveniently referred to as "DI processing". In the course of this and similar metal-forming operations a lubricant oil is applied to the surface of the metal being deformed, and some abraded aluminum particles and other contaminants (usually referred to as "smut") adhere to the metal surface, especially to the inner walls of such beverage containers. For nearly all purposes, this smut must however be removed before subsequent manufacturing operations. Thus for instance such smut must be removed from the surfaces of aluminum beverage containers before they can be satisfactorily protected by chemical-conversion coating and/or paint coating techniques. It is therefore conventional to clean aluminum articles after metal-forming operations so as to remove smut from their surfaces, and this is normally done by means of a cleaning composition which slightly etches the metal, and thus imparts a satisfactory etched appearance to the aluminum article. In this surface cleaning operation the cleaning compositions employed are normally acidic.
Till now the acidic cleaning compositions used for smut-removal have generally-speaking been ones containing chromic acid, because the use of chromic acid avoids serious problems of corrosion of the treatment apparatus. The use of compositions based on chromic acid is however nowadays avoided due to the toxicity of the chromium ion. Consequently, it has been necessary to find substitute acidic cleaning compositions; and it has been proposed to use compositions based on hydrofluoric acid. For example, according to U.S. Pat. No. 3,728,188, a cleaning agent has been proposed which consists of an acidic aqueous solution containing 0.5-2.0 g/l fluoride ion, 5-21 g/l ferric ion, and 0.05-3.0 g/l thiourea, the pH of which is regulated to 0.1-1.8 with a strong mineral acid such as sulfuric acid, etc. With this cleaner, satisfactory surface cleaning is accomplished due to the fact that the large quantity of fluoride ions causes a rapid rate of etching the aluminum, while on the other hand, this etching is inhibited by the ferric ions.
The fluoride ion however also is toxic and it is therefore still necessary to take great care to prevent pollution of the operating environment and to treat waste liquid. Of course the problems arising from the use of hydrofluoric acid are mitigated if one can reduce the concentrations of fluoride ion used; but the general experience is that with low-fluoride compositions the performance of the cleaning compositions in smut-removal is impaired.
An acidic cleaning composition has recently been developed which solves these problems, and which can achieve satisfactory cleaning despite the fact that it contains little or no fluoride ion--see co-pending United States Patent Application Ser. No. 793,019, filed Oct. 30, 1985, and assigned to the same assignee as this Application. This recently-developed acidic cleansing composition is a chromium-ion-free acid aqueous solution containing 0.2-4 g/l of ferric ions, sufficient sulfuric acid and/or nitric acid to produce a pH of 2.0 or less, and optionally also up to 0.5 g/l fluoride ions.
In this chromium-free, low- or no-fluoride cleansing composition it is thought that the etching of the aluminum by the sulfuric acid or nitric acid is promoted by the ferric ions; this etch-promotion mechanism is assumed to be cathodic reaction Fe(III)=e.sup.- --Fe (II). At all events, it has been found that in treatment baths using this new cleansing composition, the ferric ion content continuously and inevitably decreases. Hence it becomes necessary to replenish the treatment bath with a source of ferric ion in order to restore and maintain the ferric ion concentration in the treatment bath within the above-mentioned range. On the other hand, the above-mentioned cathodic reaction of the ferric ions produces ferrous ions, which tend to increase in the treatment bath. Such ferrous ions do not have an etch-promotion effect; and if they accumulate in large quantities they produce a precipitate which causes the treatment bath to become muddy and reduces its ability to perform the treatment. Furthermore, the increasing build-up of ferrous ions increases the tendency of iron to be dragged out of the treatment bath on the treated articles, and to be introduced thereby into the next subsequent chemical processing process stage, thus giving rise to an iron ion precipitate in that chemical-conversion coating stage which is detrimental to the quality of the article emergent from the final stages of the overall operations.